This invention relates to novel crystalline compositions obtained by doping the alpha (xcex1) form of aluminum phosphate ceramics.
The cristobalite phase of aluminum phosphate (AlPO4) ceramic exists in two modifications, the low temperature cristobalite (denoted as, low cristobalite, or xcex1-form) and the high temperature cristobalite (high cristobalite, or xcex2-form). The two modifications are separated by a reversible phase transformation that occurs at about 270xc2x0 C. The phase transformation results in abrupt volume and structural changes and discontinuous thermal expansion, which are not conducive to technological applications. Structures of the xcex1- and xcex2-phases have been reported by various researchers including Wright and Leadbetter (Phil. Mag. 31, 1391, 1975). AlPO4 is isomorphous with silica and exists with silica in various forms including the xcex1-cristobalite form, with phase transformations at similar temperatures. The structure of the alpha form of AlPO4 is tetragonal, a=b=5.03 Angstroms and c=7.00 Angstroms with space group C2221. The structure of the xcex1-AlPO4 phase is similar to corresponding silica structures with Al and P atoms alternately replacing the silicon atoms. (Mooney, Acta Cryst. 9, 728, 1956) The structure of xcex2-AlPO4 shows a cubic structure, space group F-43m, with a xcx9c7.2 Angstroms.
It is well known in the glass ceramics field that high temperature forms of silica can be formed at relatively low temperatures by stabilizing the ceramic matrix with dopants. The silica counterpart of the aluminum phosphate materials described above (tetragonal xcex1-cristobalite) undergoes a displacement phase transition to the cubic high temperature xcex2-phase at about 300xc2x0 C. Various reports regarding the stabilization of cristobalite phases of silica ceramics by various methods have been issued. U.S. Pat. No. 5,096,857, M. A. Saltzberg, et al., J. Amer. Ceram. Soc. 1992, 75, 89, and P. L. Gai, et al., J. Solid State Chemistry, 1993, 106, 35, all describe chemically stabilized solution-derived silica xcex2-cristobalite of the type CaxAlySi1xe2x88x92xxe2x88x92yO2 and its compositions. R. D. Shannon, et al., Phys. and Chem. Miner. 1992, 19, 157, reported compositions in the BPO4/AlPO4/SiO2 system (BAPOS), with compositions up to 75% AlPO4, 75% SiO2 and 50-60% BPO4. A relatively high amount (approximately 15%) BPO4, was used in these studies. The authors reported the presence of secondary amorphous phases (i.e., the materials were not single phase), and suggested that stabilization could be achieved using only framework ions (i.e., no ions in the interstices).
M. Rokita, et al., Pr. Kom. Nauk. Ceram. Pol. Akad. Nauk 1997, 54, 161 describe the synthesis of solid-solutions of SiO2-AlPO4. A single dopant, 20-75% mole % SiO2, was used. The structures and compositions of the solid solution with this single dopant (SiO2) were not determined because the solid solution formed multiphasic systems. Also, a relatively large mole percent (20-75 mole %) of the dopant SiO2 is used in this work. M. Handke, et al., Vib. Spectr, 1999, 19(2) 419-423 show spectroscopic data from these compounds and demonstrate that multiphasic systems are formed.
Stable ceramic materials are required for a number of end-uses, including use as piezoelectrical materials (i.e., structured materials which produce electric polarization when mechanical stress is applied), as stable supports in catalysis and biotechnology, as ceramic fillers with low dielectric constants in electronic application and as ceramic coatings for reactor materials.
In view of the foregoing, it is advantageous to develop a stable ceramic material that is single phasic through a wide range of temperatures.
The present invention relates to a stabilized AlPO4 composition comprising CaO, SiO2 and AlPO4 at a ratio of greater than 0 to less than about 4 mole percent CaO, greater than 0 to less than about 10 mole percent SiO2, greater than about 86 to less than about 100 mole percent AlPO4 The composition more preferably comprises CaO, SiO2 and AlPO4 at a mole percent ratio of greater than 0 to less than about 3 CaO, greater than 0 to less than about 6 SiO2, greater than about 91 to less than about 100 AlPO4 The composition most preferably comprises CaO, SiO2 and AlPO4 at a mole percent ratio of about 2.3 CaO, about 5.7 SiO2, about 92 AlPO4.
The synthesis involves the following steps: preparation of a slurry or sol containing the dopants, gentle drying of the slurry to drive off water and to produce amorphous precursors, and calcination to crystallize the desired phase. These synthesis procedures described below yield powders which are single phase.
The present invention also provides a method for making these compositions, comprising the steps of: admixing an acidic solution of AlPO4 to stoichiometrically appropriate solutions of SiO2 (ammonium stabilized silica sol from DuPont Ludox AS-40) and a calcium oxide source (such as calcium nitrate hydrate) wherein the mole percent ratios are greater than about 86 to less than about 100 AlPO4, greater than 0 to less than about 10 SiO2, greater than 0 to less than about 4 calcium nitrate; and the pH was adjusted to about 2.5. Calcium nitrate is an example of a convenient and economical source of CaO. The admixture is transferred to a continuous stir tank with NH4OH solution to produce a slurry with a pH of about 9. The slurry is gently heated (xcx9c70xc2x0 C.) to dehydrate and form a precipitate. The precipitates are then heated and calcined at different temperatures and X-ray diffraction measurements were performed. Subsequently, electron microscopy analyses were carried out.
Another embodiment of this invention is an AlPO4 composition that has a cubic structure, space group F-43m, with a xcx9c7.2 Angstroms at a temperature of less than about 270xc2x0 C., particularly a temperature in the range of from room temperature (approximately 25xc2x0 C.) to about 250xc2x0 C. Previously known aluminum phosphate ceramics did not have the cubic structure except after having undergone a phase change at a temperature of about 270xc2x0 C. or more.